The development of multimedia applications and the multiplicity of data networks are making data coding systems ever more complex. The interconnection of heterogeneous networks often necessitates the re-encoding, commonly referred to as transcoding, of the data streams that they convey. This re-encoding may be performed in a different compression standard from the initial compression standard (for example, re-encoding according to the MPEG-4 AVC standard (the acronym standing for “Advanced Video Coding”) for streams initially coded according to the MPEG-2 or MPEG-4 standard).
Re-encoding is also used chiefly in networks having considerable bit rate constraints. Such re-encoding systems then make it possible to tailor the bit rate of an incoming stream to the requirements and to the limitations of the network.
The regulating of bit rates used in MPEG type video coders or transcoders is undertaken by way of a regulating loop acting on the quantization step size of the DCT coefficients (DCT being the acronym for “Discrete Cosine Transform”) of the blocks of the image. The quality and the stability of this regulation depend largely on the quality of prediction of the complexities of the images to be coded for the next group of images.
In a context of source images, numerous double pass coding techniques have been developed, the first coding pass providing precise knowledge of the complexities of the images to be coded, the second pass then coding the images as a function of this complexity.
Such double pass coding techniques are very powerful but expensive in respect of transcoding systems.